In recent years, methods for increasing abrasion resistance and galling resistance of working surfaces of dies by surface-treating the working surfaces in various ways have been widely employed. Among these methods, a method that involves coating (covering) allows covering with a hard film having a Vickers hardness (HV) of higher than 1000 and provides excellent adhesion with a surface of a base material, so that a covered die obtained by applying the method has a greatly-extended life.
Practical examples of the hard film include a film of a titanium carbide (which is represented by TiC as for element symbol; the same shall apply hereinafter) deposited by a chemical vapor deposition method (CVD method), a film of VC deposited by a salt-bath method, and films of TiCN, TiC, VCN, VC, CrN, CrAlN, AlCrN, etc. deposited by a physical vapor deposition method (PVD method). Among the various covering means, the PVD method is effective in a process of covering a die, as the PVD method allows covering to be carried out at a temperature that is equal to or lower than a tempering temperature.
Moreover, with advancing enhancements in strength of workpieces, advancing increases in precision of processed products, and advancing increases in speed of molding cycles, load on the working surfaces of plastic working dies have been increased. Under these circumstances, covered dies having hard films on the working surfaces have been heavily used. Especially in the case of dies for use in cold working, TiCN and TiC have been used, as they are high in hardness and low in coefficient of friction.
Recent dies have come to be required to have excellent heat resistance so as to be able to deal with warm and hot working in addition to further increases in strength of workpieces and complication in die shapes. In view of these requirements, as a hard film deposited on a die by the PVD method, there is, for example, an AlCrSi nitride film, which is manufactured by adding a third element to the AlCr nitride film, which is excellent in abrasion resistance and oxidation resistance (Patent Literatures 1 and 2).
Meanwhile, in the field of cutting tools, a hard film is disclosed in which CrN and a nitride are laminated, the nitride being a nitride of at least one element selected from among the Group 4a elements, the Group 5a elements, the Group 6a elements, Al, and Si, which are excellent in abrasion resistance and heat resistance (Patent Literature 3).
Furthermore, as a hard film suitable for a cutting tool and a slide member, a hard film is disclosed in which a chromium nitride and VN are alternately laminated (Patent Literature 4).
A certain degree of extension of life of a die is expected by providing a hard film of any one of Patent Literatures 1 to 4 on a surface of the die. However, when used for a plastic working die, such as a press working die or a forging die, which is in a particularly harsh sliding environment, the hard film suffers significantly from wear such as abrasion or galling due to sliding of the working surface against a workpiece, and there has been room for improvement in sliding properties.
The applicants of the present application propose, as means for improving sliding properties, a covered tool whose base material has a surface covered with a hard film in which an AlCrSi nitride and a V nitride are alternately laminated, the hard film having a film thickness of 3 μm or greater, a surface roughness Ra of less than 0.2 μm, a surface roughness Rz of less than 2.0 μm, and a surface roughness Rsk of less than 0 (Patent Literature 5).